The control of feed chemicals used in the processing of liquids in a treatment system can be automated through the use of computerized control devices. Problems can occur during the automatic dosing of the chemical into the treatment system because of the inaccuracies of measurement of chemical present in the system and the variable ratio of chemical to liquid when the liquid flow rate is variable.
In the past, dosing was done by laboratory or bench testing the influent chemical concentration and measuring its flow. Subsequently, dose calculations were performed and the dosing device, a chemical feed pump for example, was manually adjusted according to the calculations. In later years, partial pacing of the dosing pumps was practiced using an influent water flow signal. However, it was still necessary to test the influent chemical concentration manually and fine tune the dosage rate of the pump manually. Still later, it was found that where the influent chemical concentration was reasonably stable and a process treatment plant, such as a wastewater plant, had a tight effluent standard to meet for environment permits for example. Varying the dose rate to maintain a specific effluent chemical concentration test result was a more direct approach that ensured better dosing results.
In recent years, reliable automatic analyzers for chemical concentration have become available enabling automation of the entire dosing procedure. Thus, the need for manual testing and manual adjusting has been practically eliminated. An additional consequence is that the automatic analyzers can also be set up to detect several important chemicals in water treatment making the dosing procedure useful for other applications such as the addition of sodium carbonate into an aerated biological reactor to control nitrification or the addition of iron or aluminum salts before a clarifier to control phosphorus removal.
One example of a water treatment system using automatic analyzers is found in an article presented by Nelu Puznava et al., on Oct. 3, 1998, entitled "Classical Feedback/Feedforward Control Applied to Methanol Dosing for Post-Denitrification" (the Puznava et al reference). The Puznava et al. reference describes a feedback/feedforward control for methanol dosing for post-denitrification in an upflow floating biofilter system. The feedback and feedforward control are based on the on-line measurement of outlet and inlet nitrate concentrations, sending two signals to the control unit. Both sensors were for chemical analysis, influent nitrate and effluent nitrate concentrations.
Japanese Patent No's. Sho 52-93160 and Sho 51-130055 to Tokyo Shibaura Electric Co. (the '160 ad the '055 references) both relate to an apparatus for control of the feed rate of water purification reagents. The apparatus consists of a source water quality measurement meter for measurement of water quality of the source water intake, a reagent feed device, a ratio setting device that maintains a ratio of the reagent feed rate to the source water intake, a settling water quality measurement meter that measures the water quality of settling water and outputs a signal, and a calculating control device that receives the output signals and sets the flow rate of the reagent and sets the ratio setting device. The apparatus measures water quality factors: source water turbidity, pH, alkalinity and temperature, not concentration of the reagents.
U.S. Pat. No. 4,425,291 to Matsko (the '291 reference) discloses a system for controlling the dosing of chlorine in a system for chlorinating waste water. In the '291 reference, the chlorine dosage is controlled by electronic controllers according to a derivative of residual chlorine with respect to chlorine dosage to provide an accurate control of chlorine to insure oxidation of ammonia in waste water. Flow transmitters sense the flow of chlorine, base or sulfur dioxide to their respective tanks.
U.S. Pat. No. 4,544,489 to Campbell et al (the '489 reference) discloses a process and apparatus for the controlled addition of a conditioning polymer material to sewage sludge. The '489 reference employs a computer with a connected viscometer. Based upon the shear stresses measured and input to the computer by the viscometer, the system controls the rate of pumping of the polymer to mix with the sludge.
None of the references teach or suggest a method for automatic dose control of liquid treatment chemicals used for processing liquids within a liquid treatment system that accurately measures the correct amount of chemical required in a varied liquid flow rate system. Consequently, there remains a need for accuracy in the method for measuring chemicals in both the influent flow and the effluent flow of a variable flow system. Further, there is a need for a method for automatic dose control of liquid treatment chemicals with enhanced control to improve the chemical dosing during water treatment.